As a high-speed wireless access system using a band of 5 GHz, there is an Institute of Electrical and Electronics Engineers (IEEE) 802.11a standard. In this system, an orthogonal frequency division multiplexing (OFDM) modulation scheme, which is technology which stabilizes the performance in a multipath fading environment, is used and a maximum throughput of 54 M bits per second (bps) is realized. However, because the throughput here is throughput on the physical layer and transmission efficiency in a medium access control (MAC) layer is actually about 50 to 70%, the upper limit of the actual throughput is about 30 Mbps (e.g., see Non-Patent Document 1).
Further, IEEE 802.11n is aimed at realization of high-speed communication by: multiple-input multiple-output (MIMO) technology capable of realizing spatial multiplexing in the same frequency channel at the same time using a plurality of antennas; technology using a frequency channel of 40 MHz by simultaneously employing two frequency channels of 20 MHz which are separately used so far; and/or technology which improves efficiency such as frame aggregation in which a plurality of frames are bundled and transmitted and reduction in overhead of a control signal by using a block acknowledge (ACK) signal, and it is capable of realizing a maximum transmission speed of 600 Mbps (e.g., see Non-Patent Document 1).
In addition, recently, the demand for wireless communication has rapidly increased and access points of a wireless LAN have been installed in many places. However, there is a problem in that signals of nearby communication cells interfere with each other and excellent wireless communication cannot be realized in an environment in which the communication cells (each configured by one access point and a plurality of stations) are close to each other (in general, in a wireless communication system of a mobile phone, a wireless LAN, or the like, one access point or a communication cell including an access point and a plurality of stations is regarded as a minimum unit of a wireless network).
To address this problem, technology (cooperative transmission technology using interference suppression technology) for increasing the throughput in which each of a plurality of access points uses a plurality of antennas mounted on each access point, controls directivity of radio waves (transmission beamforming) by changing a phase rotation amount of a signal transmitted from each antenna, and performs communication of the access point itself while suppressing interference to nearby communication cells has been studied (e.g., see Non-Patent Document 2). In the interference suppression technology based on the transmission beamforming, interference suppression is performed by acquiring a propagation channel between an access point itself and an interfering station in advance, calculating transmission weights which suppress the interference from the propagation channel, and performing communication using the transmission weights. Here, the propagation channel represents a received intensity and a phase rotation amount of a signal when radio waves are propagated from transmit antennas to receive antennas.
FIG. 21 is a block diagram showing a configuration of a general wireless LAN transmitter capable of performing the transmission beamforming. As shown in FIG. 21, the wireless LAN transmitter is configured by error-correction coding units 100-1 to 100-A, interleaving processing units 101-1 to 101-A, subcarrier modulating units 103-1-1 to 103-B-A, a weighting processing unit 104, inverse fast Fourier transform (IFFT) units 105-1 to 105-C, guard interval (GI) adding units 106-1 to 106-C, radio frequency (RF) processing units 107-1 to 107-C, antennas 108-1 to 108-C, a preamble generating unit 109, a pilot subcarrier generating unit 110, a wireless signal demodulating unit 111, a propagation channel acquiring unit 112, and a weight calculating unit 113.
The error-correction coding units 100-1 to 100-A perform convolution encoding of input data. The interleaving processing units 101-1 to 101-A rearrange bits so that transmission of adjacent bits after encoding is performed in subcarriers separated as much as possible. The subcarrier modulating units 103-1-1 to 103-B-A modulate data on which an interleaving process has been performed in accordance with a modulation scheme such as binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) prescribed in a wireless LAN standard.
The weighting processing unit 104 multiplies the data by transmission weights calculated from the propagation channel for subcarriers. The IFFT units 105-1 to 105-C transform frequency-series data on which the weighting has been performed into time-series data using IFFT computation. The GI adding units 106-1 to 106-C are blocks which copy a fixed period in a rear end of an IFFT output signal and connect it to a front end of the IFFT output signal. Each of the RF processing units 107-1 to 107-C converts a baseband signal to which a GI has been added into a wireless signal using an analog RF apparatus. Each of the antennas 108-1 to 108-C radiates the wireless signal into air.
The preamble generating unit 109 generates preamble signals used to perform timing synchronization and frequency synchronization of a wireless signal configured by known signals (the preamble signals and pilot subcarrier signals). The pilot subcarrier generating unit 110 generates the pilot subcarrier signals which are configured by known signals and are used to correct a residual frequency error. The wireless signal demodulating unit 111 demodulates a wireless signal transmitted from a station and acquires a data portion included in the wireless signal. The propagation channel acquiring unit 112 acquires a propagation channel for the subcarriers acquired from the demodulated data portion and stores the propagation channel. The weight calculating unit 113 calculates the transmission weights using the propagation channel acquired for the subcarriers.
As shown in FIG. 21, the wireless LAN transmitter converts transmission data in various blocks and thus it is possible to generate and transmit a wireless LAN signal in which transmission beamforming is possible.
In addition, MIMO transmission used in IEEE 802.11n is performed between an access point (AP) and a station (STA) which face with each other, and it improves the throughput by distributing data to be transmitted to a plurality of antennas and transmitting the data in parallel with each other, i.e., by spatially multiplexing, from the plurality of antennas.
Furthermore, in IEEE 802.11ac under development at present, research on technology called multiuser (MU)-MIMO in which an access point and a plurality of stations perform one-to-many communication by spatially multiplexing using the same wireless channel, thereby making it possible to effectively use the wireless space resources is ongoing as a wireless system to which the MIMO transmission is applied (e.g., see Non-Patent Document 3). In the MU-MIMO, a wireless access point performs communication through spatial multiplexing by spatially separating data packets addressed to a plurality of wireless stations using beamforming, to thereby improve the throughput.
Here, the MU-MIMO transmission will be specifically described with reference to the drawings.
FIG. 22 is a block diagram showing a configuration of an MU-MIMO transmission system. The communication system shown in FIG. 22 is provided with an access point 1110 and stations 1111 and 1112 which perform wireless packet communication with the access point 1110. In addition, H1 and H2 represent propagation channels.
FIG. 23 is a time chart describing an operation of the MU-MIMO transmission. As shown in FIG. 23, in the MU-MIMO transmission, frames include a carrier sense (CS) which checks whether another wireless device is performing communication, a null data packet announcement (NDPA) which announces transmission of a null data packet, a null data packet (NDP) configured by null data, a beamforming report (BR) which notifies of propagation channel information estimated from the NDP, a beamforming report poll (BRP) which requests propagation channel information, data (Data1 and Data2) for the station 1111 and the station 1112, a block acknowledgement (BACK) which notifies of whether a signal has been accurately received, and a block acknowledgement request (BACKR) which requests the block ACK.
It is assumed that data (transmission target data) of a packet to be transmitted to the stations 1111 and 1112 have been generated in the access point 1110. Accordingly, the access point 1110 executes a carrier sense (CS) at random time intervals. As a result of the carrier sense, it is determined whether the state is an idle state in which a communication frequency band is not used or a busy state in which the communication frequency band is used.
For example, it is assumed that as a result of the carrier sense (CS) executed at time t101, the idle state, in which the communication frequency band is not used, have been detected. Accordingly, the access point 1110 generates and transmits an NDPA in a period from, for example, time t103 at which a certain time has elapsed from time t102 to time t104.
Next, the access point 1110 generates and transmits an NDP for estimating a propagation channel in a period from time t105 at which a certain time has elapsed from time t104 to time t106. In this case, the access point 1110 recognizes the stations 1111 and 1112 of destinations of the above transmission target data. Then, the access point 1110 designates the stations 1111 and 1112 as the destinations and transmits a signal for measurement.
In response to reception of the above signal for measurement, the stations 1111 and 1112 measure the performances of propagation channels within the same period from time t105 to time t106. Then, the stations 1111 and 1112 generate BRs including the performances of the propagation channels or information calculated from the performances of the propagation channels in a period from time t106 to time t107.
Next, the station 1111 transmits the BR in a period from time t107 at which a certain time has elapsed from time t106 to time t108.
Next, the access point 1110 generates and transmits a BRP which requests the station 1112 to transmit propagation channel information in a period from time t109 at which a certain time has elapsed from time t108 to time t110.
Next, the station 1112 transmits the BR in a period from time t111 at which a certain time has elapsed from time t110 to time t112 in response to reception of the above BRP.
Next, the access point 1110 calculates transmission weights and generates a transmission signal using the notified BRs. In addition, the access point 1110 transmits transmission target data in a period from time t113 at which a prescribed time has elapsed from time t112 to time t114. It is to be noted that the data to be transmitted in the period from time t113 to time t114 is, for example, converted into a frame suitable for wireless communication. In addition, when the frame aggregation has been applied, the data to be transmitted in the period from time t113 to time t114 is a data unit in which a predetermined number of frames are connected.
Then, in response to completion of reception of the data at time t114, the station 1111 transmits a BACK in a period from time t115 at which a certain time has elapsed from time t114 to time t116. The access point 1110 receives the BACK and executes a predetermined process corresponding to reception of the BACK. Specifically, for example, the access point 1110 determines that a receiving end has received the data normally by reason of reception of the BACK and transitions to a process of transmitting and receiving the next data. In addition, if timeout has been reached without receiving the BACK, the access point 1110 executes a process such as retransmission of the transmission target data.
Next, the access point 1110 generates and transmits a BACKR for requesting the station 1112 to transmit a BACK in a period from time t117 at which a certain time has elapsed from time t116 to time t118.
Next, the station 1112 transmits the BACK in a period from time t119 at which a certain time has elapsed from time t118 to time t120. The access point 1110 receives the BACK and executes a predetermined process corresponding to reception of the BACK.
In Non-Patent Document 3, MU-MIMO transmission is performed in accordance with the above time chart.